Aluminum alloy compression type connectors for use with aluminum or copper conductors

ABSTRACT

Electrical connectors are constructed by forming a tubular aluminum alloy casting. The tubular casting is warm worked into a tube, cold drawn to size, and then heat treated to provide the desired heat stable tensile and elongation properties. The heat treated tube is then cut into sections and one end of each section is compressed in a confined volume to a predetermined pad-like configuration. The other end of the heat treated section forms the barrel portion of the connector. The compression or cold working of the one end which is to be the pad portion of the connector considerably hardens and strengthens the metal so that it more nearly matches that of the bus or other electrical terminal equipment to which it will be secured. The barrel end remains somewhat softer, thereby more nearly matching the physical characteristics of the stranded electrical wire conductors. This connector, made of heat stable aluminum alloy, is seen to have different tensile strengths in the barrel and the pad portions. The barrel portion has an ultimate tensile strength of between 16,000 pounds per square inch and 19,000 pounds per square inch while the ultimate tensile strength of the pad portion, due to the cold working, exceeds 22,000 pounds per square inch (p.s.i.). The alloy is selected to have an electrical conductivity that exceeds 60% of that of copper, i.e.e, 60% of that required by the International Annealed Copper Standard. When the alloy is properly processed prior to actually forming the connector, it has an ultimate tensile strength between 16,000 and 19,000 p.s.i. and an elongation of more than 10%. In addition, the alloy preferably should be heat stable and capable of maintaining its properties within 10% during temperature variation of 0* to 400*F. for periods of up to 4 hours and more. All of these physical characteristics go to make a unique connector that is capable of use with either aluminum or copper electrical conductors and bus without many of the attendant disadvantages inherent in existing aluminum connectors.

United States Patent [191 Miller et al.

[4 1 Oct. 14, 1975 ALUMINUM ALLOY COMPRESSION TYPE CONNECTORS FOR USE WITH ALUMINUM OR COPPER CONDUCTORS [76] Inventors: Roger D. Miller, 141 Senaca Drive,

Noank, Conn. 06340; Maurice M. Gilbert, 306 Taylor Lane, Kennett Square, Pa. 19348 [22] Filed: Nov. 19, 1974 [21] Appl. No.: 525,179

Related US. Application Data [63] Continuation of Ser. No. 371,475, June 19, 1973, abandoned, which is a continuation-in-part of Ser. No. 213,339, Dec. 29, 1971, abandoned.

[52] US. Cl. 339/276 T; 29/629; 29/630 A; 174/84 C; 174/90 [51] Int. Cl. H02G 15/08 [58] Field of Search 174/84 C, 90, 94 R, 75 R, 174/84 R; 339/97 C, 276 R, 276 T, 278 C, 277 R; 29/630 A, 628, 629

OTHER PUBLICATIONS Alloy Digest, Aluminum EC Filing Code, AL-l04 June 1961, 2 pp. published by Engineering Digest, lnc., Upper Montclair, NJ.

Primary ExaminerDarrell L. Clay Attorney, Agent, or Firm-C. Walter Mortenson [5 7 ABSTRACT Electrical connectors are constructed by forming a tubular aluminum alloy casting. The tubular casting is warm worked into a tube, cold drawn to size, and 'then heat treated to provide the desired heat stable tensile and elongation properties. The heat treated tube is then cut into sections and one end of each section is compressed in a confined volume to a predetermined pad-like configuration. The other end of the heat treated section forms the barrel portion of the connector. The compression or cold working of the one end which is to be the pad portion of the connector considerably hardens and strengthens the metal so that it more nearly matches that of the bus or other electrical terminal equipment to which it will be secured. The barrel end remains somewhat softer, thereby more nearly matching the physical characteristics of the stranded electrical wire conductors.

This connector, made of heat stable aluminum alloy, is seen to have different tensile strengths in the barrel and the pad portions. The barrel portion has an ultimate tensile strength of between 16,000 pounds per square inch and 19,000 pounds per square inch while the ultimate tensile strength of the pad portion, due to the cold working, exceeds 22,000 pounds per square inch (p.s.i.). The alloy is selected to have an electrical conductivity that exceeds 60% of that of copper, Lee, 60% of that required by the lntemational Annealed Copper Standard. When the alloy is properly processed prior to actually forming the connector, it has an ultimate tensile strength between 16,000 and 19,000 p.s.i. and an elongation of more than 10%. In addition, the alloy preferably should be heat stable and capable of maintaining its properties within 10% during temperature variation of 0 to 400F. for periods of up to 4 hours and more. All of these physical characteristics go to make a unique connector that is capable of use with either aluminum or copper electrical conductors and bus without many of the attendant disadvantages inherent in existing aluminum connectors.

5 Claims, 5 Drawing Figures U.S.'Patent 0t.14,1975 sheetlofz 3,912,358

Form/ Tubular Alloy Caasiizg Work Gas [Ilia T ube Tube A ma Treaimerzt Gui Into Seciions l D is Form Shaped] Cold Draw I Gonzzeaior Pad Usirgfiarrellns'eri Paw/L Holes Clea/z h laie INVENTOES Boyer D. M iller MmzriceM Gilbert TTZIOJZNEYST US. Patent Oct. 14,1975 Sheet2of2 3,912,358

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- BoyerD. M iller MaambcllLGiZberi BY zg omv W ALUMINUM ALLOY COMPRESSION TYPE CONNECTORS FOR USE WITH ALUMINUM OR COPPER CONDUCTORS BACKGROUND OF THE INVENTION This application is a continuation of Ser. No. 371,475 filed June 19, 1973 entitled Aluminum Alloy Compression Type Connectors For Use With Aluminum or Copper Conductors, now abaondined, which is a continuation-in-part of a patent application Ser. No. 213,339 filed Dec. 29, 1971 by Roger D. Miller and Maurice M. Gilbert and now abandoned Over the last several decades aluminum has come into greater and greater use in electrical conductors and connectors due in no small part to the continually increasing prices of and often questionable availability of copper. On the other hand, because the physical properties of aluminum are appreciably different than those of copper, numerous problems and electrical failures have occurred, often at the weakest point in the entire system the connectors.

Among the advantageous properties of aluminum are its light weight, ready availability and low cost. On the other hand, aluminum has many other properties which place it at a distinct disadvantage in comparison to copper, particularly when used in connection with copper conductors. These disadvantageous properties of aluminum are high cold flow, high coefficient of thermal expansion, susceptibility to galvanic corrosion, and the formation of a very tenacious insulating oxide coating on its surface. The usual method of overcoming the problem of the insulating oxide coating and galvanic corrosion in the presence of copper is to clean and plate the surface of the aluminum connector with a less chemically active material. To prevent further corrosion, the conductor strands are normally coated with an anti-oxide grease and the internal portion of the connector barrel is also coated with this same material before the connector is compressed onto the conductors. This anti-oxide grease prevents moisture from entering the connector and causing galvanic corrosion. It is also desirable that the surface of the connector pad where it is bolted to the bus be coated with an anti-oxide grease and bolted very tightly for the same purpose. Unfortunately, the cold flow characteristics and expansion coefficient of aluminum are not conducive to extremely tight bolting.

When aluminum connectors first came into use, mainly since 1958, most connector manufacturers simply used the physical dimensions of the existing copper connectors for aluminum. The only concession to the use of the new aluminum material was that the barrel portion of the connector was formed to provide a heavier wall thickness. Relatively soft aluminum alloy materials were used such that existing tools used with copper connectors would also be used with the aluminum material. Furthermore, since aluminum material was relatively.cheap, the conductors employed were often oversized. This permitted a connector-wire assembly that operated at relatively low temperatures and few problems were encountered.

As the use of aluminum connectors grew and was extended into various terminal and switching equipment having restricted space requirements, particularly as manufacturers began to reduce the size of equipment,

in an effort to reduce their costs, the required smaller size connectors soon underwent many failures. These failures were caused to a large extent by the hotter operating temperatures created thereby.

Since the aluminum used in the manufacture of these connectors is relatively soft, it tends to cold flow and creep away from the high pressure area of the bolts or compression tool. Thus, as the connector goes through successive heating and cooling cycles, due not only to atmospheric changes but more probably from the heat generated by the current flow, the connection becomes loose and has increased electrical resistance. This increased electrical resistance increases the possibility of galvanic corrosion when used with copper conductors and, of course, ultimately can cause failure due to arcing at the connection. This arcing can be relieved to some extent by the utilization of compression springtype washers normally known as Bellville washers. However, these type washers take longer to install and in themselves negate many of the economic advantages of utilizing aluminum in the first instance.

Arcing at the points of contact is further aggravated by the relatively high coefficient of thermal expansion of aluminum as compared to the copper bus and the bolts used in the joints. The repeated heating and cooling during utilization over a period of time causes the connector and the bus to expand and contract and if the connector pad is not made of the proper alloy, it will deform and reduce interface pressure between the connector pad and the bus. Thisusually results in a high resistance joint and ultimate failure.

An additional problem is encountered even when aluminum conductors are employed with aluminum connectors. Most of the aluminum compression connectors are made from a soft aluminum (1 -0) while the aluminum conductors have a much higher tensile strength (and hence are harder the most common being ECH-l9 having a tensile strength of at least 22,000 p.s.i.), usually about twice as much as the tensile strength of the connectors. This prevents the force that is used to compress the connector about the conductors from being distributed uniformly through the various strands of the conductors and only the outside strands of the conductor are deformed sufficiently to penetrate the inherent aluminum oxide coat and make intimate contact with the aluminum connector. This obviously results in a higher resistance between the conductor and the connector and eventually failure will occur.

It is, therefore, an object of this invention to obviate many of the disadvantages of the prior art aluminum electrical connectors.

A further object of this invention is to provide an improved aluminum alloy electrical connector that can be used with either copper or aluminum conductors and is free of most of the disadvantages of prior art aluminum connectors.

Another object of this invention is to provide an improved method for making aluminum alloy electrical connectors having improved electrical mechanical characteristics.

A still further object is to provide an improved connector having a barrel which when compressed onto strands of aluminum wire, each strand of the wire will be substantially uniformly deformed to provide substantially equal distribution of current in the strand thereby permitting cooler junctions between the wire and connector.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT In accordance with the preferred method of this invention an electrical connector is formed of an aluminum alloy for connecting electrical wire conductors to electrical equipment such as terminals, buses or for splicing electrical wire conductos. This aluminum alloy is selected to have an initial ultimate tensile strength prior to the heat treatment of 20,000 p.s.i. This alloy is processed by heat treatment, cold drawn into tubes which are further heat treated to have the desired physical, mechanical and electrical characteristics. After it has been established that the tube has the proper characteristics, the tube is cut into the desired lengths and one end is compressed in a confined volume of a predetermined configuration to form a connector pad. During the forming of the pad it is preferable to use a specially designed insert, otherwise the connector cannot be formed. Such operation work hardens substantially only the pad portion of the connector such that the pad has characteristics that are more compatible with the typical characteristics of the bus or other electrical terminal equipment to which it is attached and whereby the barrel portion of the connector has characteristics which are more compatible with the electrical conductors it is to receive.

The connector thus formed has a barrel portion having an ultimate tensile strength of 16,000 to 19,000 p.s.i. and a pad portions whose ultimate tensile strength exceeds 22,000 p.s.i. The electrical conductivity of the alloy is selected such that it exceeds 60% of that of the International Annealed Copper Standard. The connector has a minimum yield strength of 14,000 p.s.i., an elongation that exceeds 10%, and a heat stability such that the ultimate tensile strength, elongation and yield strength do not change more than 10% over the temperature range of to 400 F. for periods of time up to 4 hours. Thus formed, the connector may be used with either copper or aluminum conductors and may be used on either copper or aluminum buses or terminals using either aluminum or steel bolts and does not require the utilization of compression washers in order to maintain a suitable electrical contact particularly during heat cycling.

BRIEF DESCRIPTION OF THE DRAWINGS The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention, itself, however, both as to its organization and method of operation as well as additional objects and advantages thereof, will be best understood from the following description when read in connection with the accompanying drawings in which:

FIG. 1 is a flow diagram illustrating the steps involved in the method of this invention;

FIG. 2 is a side elevation view of a piece of tubing used to make the connector of this invention prior to its being processed with the insert in position;

FIG. 3 is a plan view of the connector of this invention during processing immediately after the pad has been formed;

FIG. 4 is a side elevation view of the connector illustrated in FIG. 3 taken along the section lines 4-4, illustrating a forming insert used in the formation of the connector barrel, and

FIG. 5 is an end view of the connector.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The preferred method of this invention envisions forming an electrical compression-type connector 10, illustrated in FIGS. 2 through 5, inclusive, having a hollow cylindrical barrel end 12 and a flattened end or pad 14. The pad 14 has holes 16 punched therein to accommodate the pad 14 being bolted to a terminal or bus. The pad 14 preferably is rectangular in cross-section and is dimensioned to provide the proper current density within the pad, proper current density in interface between'connector and bus, provide the proper tensile strength, as well as to be compatible with the standard bus or other electrical terminal equipment as specified by the National Electrical Manufacturing Association (NEMA).

The connector preferably is formed of an aluminum alloy having certain physical characteristics which include an electrical conductivity of not less than 60%, i.e., 60% of the IACS, an initial ultimate tensile strength in excess of 16,000 p.s.i., a minimum yield strength of at least 14,000 p.s.i. and a minimum elongation of 10%. The alloy also should be heat stable, i.e., it should have substantially the same mechanical characteristics after being subjected to temperatures up to 400 F. for up to 4 hours. By substantially the same properties it is meant that the properties do not change more than a maximum of 10% from the values specified. Preferably, the properties of the materials should change less than 5% during such temperature cycling.

The word copper as used in this description and claims is defined as having a volume conductivity at 20 C. (68F) of IACS. This means that a copper wire 1 meter in length and weighing 1 gram will have a resistance of 0.15328 ohms. When aluminum is described as having a conductivity of 60% (IACS), it is the same as saying an aluminum wire 1 meter in length and weighing 1 gram will have a resistance at 20 C. (68F.) of (0.60 X 0.15238) ohms. Elongation is defined as the percentage increase in distance between two gauge marks that results from stressing the specimen under test to fracture. The original gauged length is usually based on 10 inches for wire up to one-quarter inch in diameter, and 2 inches for sheet specimens and round specimens whose diameter is one-half inch or four times the diameter for specimens where that dimension is under one-half inch. The ultimate tensile strength is, by definition, the tensile strength of a material at the time of rupture. The yield strength is the unit stress at which a material exhibits a specified limiting permanent set, which for aluminum is usually taken as 0.2%.

Heat stable aluminum alloys having these physical characteristics may be obtained from several sources. One such source is Southwire Company at Carrollton, Geo. They have two products which meet these specifications one being sold under the trademark Triple E and the other being sold under the trademark Super T. The Triple E alloy sold by Southwire includes the following metals: iron 0.60%, silicon 0.05%, magnesium 0.002% manganese 0.002%, copper 0.004%, boron 0.006% and trace amounts of several other elements. This Triple E alloy has a particular advantage in that it is relatively heat stable and the tensile strengths specified above are essentially maintained within the range of temperature variations to 400 F., the maximum that can occur during normal elec'trical design conditions. Typically, the tensile strengths vary less than 5% within such range of temperature variations. An additional quality of this particular aluminum alloy is that at temperatures below 400 F. there is relatively little or no variation in the hardness of the metal. This high degree of hardness or high tensile strength tends to reduce the cold flow of the metal. Other aluminum alloy sources having the proper physical and electrical characteristics are Alcoas C K-76, its chemical composition being about 0.6% to about 0.9% iron, about 0.08% to about 0.22% magnesium and the balance aluminum, or Anacondas alloy sold under the trademark i430 Analoy.

The heat stable aluminum alloy connector described herein having a pad portion which is harder and which has. a higher tensile strength than the barrel portion may be constructed according to a preferred method of this invention by first forming a tubular aluminum casting from an aluminum alloyhavingiat least one alloy element taken from the group consisting of copper, manganese, silicon, magnesium, zinc,'boron, and iron. The alloy casting is worked into a tube and then cold drawn using conventional aluminum drawing techniques. As the next step thedrawn tube is then heat treated at varying temperatures between 470 F. and 540 F. and a length of time depending upon its initial temper to secure the proper working physical properties set forth above. With a higher initial temper, the higher temper atures are used and conversely with a lower initial temper. The time may be varied somewhat times of 3 hours tone-half hour may be used with facility, although 3 hours is the preferred time. In a given case, the time may be shortened as the temperature is increased and vice versa.

In typical examples, a Triple E aluminum alloy from Southwire having an initial hard drawn temper and initial ultimate tensile strength of 2l-23,000 p.s.i., heat treated 3 hours at 485F. exhibits a final ultimate tensile strength of l8,000 p.s.i. In another example, a Triple E aluminum alloy having a hard drawn temper and initial ultimate tensile strength of 2l-23,000 p.s.i., heated treated for 3 /2 hours at 525. F. exhibits a final ultimate tensile strength in the range of 16,000 p.s.i.

The tubing is then cut into sections approximating the length of each connector to be formed. A tube section is then placed in a press and die and a small forming insert tool 18 (FIG. 4) placed in one end of the tube at a position immediately adjacent the point where the barrel portion 12 terminates. This prevents the shearing or collapse of the tube portion between thepad and barrel. Surprisingly, and in contrast to copper connectors, the connector cannot be formed without the use i of thistool. The press then is activated to compress the remaining end of the tube 20 in a contained volume corresponding to the cross-section of the desired connector pad portion 14 the contained volume being provided by the die. This contained volume is such that the material in the pad is cold worked in both the pad thickness and width dimensions. Preferably the pad thickness is less than twice the wall thickness of said tubing. Most preferably the pad thickness is about 20% greater than a single wall thickness of the tubing, i.e., once the pad width is selected for a specific connector, typically 25-30% of the material must be displaced to secure the required hardness. Such degree of cold working producesa pad having an ultimate tensile strength of more than 22,000 p.s.i. The junction between the barrel and pad is formed about the insert tool 18 as noted.

By thus cold working the tube to form the flat pad portion l4, the tensile strength of the pad portion is greatly increased so as to render the pad portion more compatible with the hardness of the normal bus materials employed in electrical equipment whether aluminum or copper. Preferably, this work hardening of the pad raises its ultimate tensile strength to more than 2 2,000 p.s.i. The insert 18 is now removed. Holes 16 are now punched in the pad to accommodate a bolt or bolts to bolt the bus or other terminal equipment. The entire connector is suitably cleaned and plated preferably by nickel although any other suitable materials such as tin or silver may be used. Nickel provides additional surface hardness and reduces the interface electrical resistance. Since the nickel plating preferably completely covers all surfaces of the connector including the inside of the barrel, the entire connector surface is rendered inert to oxide formation, and electrolysis when used with either copper wire and/or copper bus.

The pad of the conductor according to the method of this invention, is formed, as described, to have a minimum ultimate tensile strength of 22,000 p.s.i. or higher and hence is more able to withstand the bolt tightening and other strains and stresses described hereinbefore.

. A connector of this type made of a heat stable aluminum alloy having the noted characteristics and having a: pad portion with such.- a higher ultimate tensile strength than, the barrel portion has many advantages. With the harder, stronger, heat stable aluminum alloy, greater wire strand deformation can be achieved during crimping of the barrel resulting in improved contact between the wire strands and hence better conductivity and current balance at the connector. The reason for this is that the non-conducting aluminum oxidecoating, which normally forms on the wire strands, is more easily penetrated since with the harder connector material, the greater the pressure that can be applied to each conductor strand with the usual barrel crimping tools. With enhanced and more uniform wire strand deformation far lower resistance and more uniform current density among the strands of the conductor is achieved, as noted, due to the penetration of the nonconducting aluminum oxide coating that forms on aluminum conductor strands. Due to its heat stability, little or no tensile or hardness changes occur within design operating limits up to 400F., the connector maintains a high tensile strength and an elongation which permits the maintenance of connection pressure even during operational heat cycling induced by normal current changes.

Another significant advantage is that the work hardened connector pad, being more compatible with conventional aluminum or copper buses or other electrical terminal equipment, allows installation using conventional tools, bolts, and flat washers rather than the torque wrenches and/or Bellville washers that are required with most aluminum connectors. Also, the hardened pad of the connector maintains pad contact pressure with little if any cold flow during operation even with severe temperature changes.

In alternative methods two or more connectors depending on size of connector and capacity of press may be punched at once in a punch and die set up. They may be simultaneously cut into individual connectors with the punching operation. Further, the last treating and cutting steps may be reversed as desired.

There has thus been described a unique connector and method for producing the same which provides an improved interface between aluminum electrical conductors and electrical terminal equipment which interface has the same high degree of reliability which now exists with copper systems. Copper connectors having greater hardness and lower cold flow than aluminum connectors of existing art produced far less electrical failures which often occur due to the current cycling during use of a connector. The heat stable aluminum alloy connectors described herein provide many of the same advantages that were previously only available from copper.

It is obvious that many embodiments may be made of this inventive concept, and that many modifications may be made in the embodiments hereinbefore described. Therefore, it is to be understood that all descriptive material herein is to be interpreted merely as illustrative, exemplary and not in a limited sense. It is intended that various modifications which might readily suggest themselves to those skilled in the art be covered by the following claims as far as the prior art permits.

What is claimed is:

l. A crimpable electrical connector for connecting electrical conductors to electrical terminal equipment comprising:

a body formed of an aluminum alloy having a yield strength of more than 14,000 pounds per square inch,

an elongation of more than 10%, an electrical conductivity that exceeds 60% IACS, and a heat stability such that the tensile strength, elongation and yield strengths do not change more than 10% over the temperature range of 0 to 400 F. for periods of time up to 4 hours, said body having a pad portion adapted to contact said electrical terminal equipment and a barrel portion adapted to receive said conductors, said barrel portion having a tensile strength of more than 16,000 pounds per square inch, and said pad portion having a higher tensile strength than said barrel portion whereby said pad portion is capable of maintaining improved contact pressure with said electrical equipment and said barrel portion is capable of maintaining good electrical contact with substantially all of said conductors, said aluminum alloy containing from 0.6% to 0.9% iron. 2. An electrical connector in accordance with claim 1 in which said iron content is 0.6%.

3. An electrical connector in accordance with claim I in which said aluminum alloy is plated with a metal selected from the group consisting of silver, nickel and tin.

4. An electrical connector in accordance with claim 1 in which said pad portion has a tensile strength of more than 22,000 pounds per square inch.

5. An electrical conductor in accordance with claim 1 in which said change as to tensile strength, elongation and yield strengths is not more than 5% as to each said value. 

1. A CRIMPABLE ELECTRICAL CONNECTOR FOR CONNCTING ELECTRICAL CONDUCTORS TO ELECTRICAL TERMINAL EQUIPMENT COMPRISING: A BODY FORMED OF AN ALUMINUM ALLOY HAVING A YIELD STRENGTH OF MORE THAN 14,000 POUNDS PER SQUARE INCH, AN ELONGATION OF MORE THAN 10%, AN ELECTRICAL CONDUCTIVITY THAT EXCEEDS 60% IACS, AND A HEAT STABILITY SUCH THAT THE TENSILE STRENGTH, ELONGATION AND YIELD STRENGTHS DO NOT CHANGE MORE THAN 10% OVER THE TEMPERATURE RANGE OF 0* TO 400*F. FOR PERIODS OF TIME UP TO 4 HOURS, SAID BODY HAVING A PAD PORTIN ADAPTED TO CONTACT SAID ELECTRICAL TERMINAL EQUIPMENT AND A BARREL PORTION ADAPTED TO RECEIVE SAID CONDUCTORS, SAID BARREL PORTION HAVING A TENSILE STRENGTH OF MORE THAN 16,000 POUNDS PER SQUARE INCH, AND SAID PAD PORTION HAVING A HIGHER TENSILE STRENGTH THAN SAID BARREL PORTION WHEREBY SAID PORTION IS CAPABLE OF MAINTAINING IMPROVED CONTACT PRESSURE WITH SAID ELECTRICAL EQUIPMENT AND SAID BARREL PORTION IS CAPABLE OF MAINTAINING GOOD ELECTRICAL CONTACT WITH SUBSTANTIALLY ALL OF SAID CONDUCTORS, SAID ALUMINUM ALLOY CONTAINING FROM 0.6% TO 0.9% IRON.
 2. An electrical connector in accordance with claim 1 in which said iron content is 0.6%.
 3. An electrical connector in accordance with claim 1 in which said aluminum alloy is plated with a metal selected from the group consisting of silver, nickel and tin.
 4. An electrical connector in accordance with claim 1 in which said pad portion has a tensile strength of more than 22,000 pounds per square inch.
 5. An electrical conductor in accordance with claim 1 in which said change as to tensile strength, elongation and yield strengths is not more than 5% as to each said value. 